1. Field of the Invention
This invention relates to the shaping of heat-softened glass sheets and, in particular, to shaping glass sheets between a full surface press face and a sheet supporting conveyor belt.
2A. Technical Consideration
Shaped glass sheets are widely used as windows in vehicles such as automobiles and the like. To be suitable for such applications, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the window frame. It is important that these windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with clear viewing therethrough in the viewing area.
One commercial method of shaping a glass sheet is to convey the glass sheet through a tunnel type heating furnace to heat the glass sheet to its heat softening temperature and pressing the glass sheet between a pair of contoured shaping molds. Another commercial operation which is used, in particular to simultaneously shape overlying glass sheets or doublets for glass windshields, supports the flat doublets on the contoured shaping rails of a bending iron as they are conveyed through a heating lehr. As the temperature of the glass sheets increases and approaches the heat softening temperature, the glass begins to sag under the force of gravity and the glass sheets' peripheral edge conforms to the contours of the rails. In order to properly shape the central portion of the glass doublet by gravity sag bending, the temperature of the glass sheets must be precisely controlled.
2B. Patents of Interest
U.S. Pat. No. 3,806,330 to Martin teaches a vertical press bending operation wherein a glass sheet is suspended within a refractory fabric sack supported along its upper edge while heating the glass to a desired softening temperature. After heating, the sheet and sack are transported to a position between opposing press faces which sandwich the heat softened glass sheet and sack therebetween to shape the glass to the desired configuration.
U.S. Pat. No. 4,318,728 to Claassen teaches the shaping of a glass sheet between pressing molds of complementary curvature while the glass sheet is continuously moved by a belt from a furnace exit to a cooling area. The movement of the molds is synchronized with the continuously moving glass sheet to simultaneously move toward and then away from each other and sandwich the glass sheet and belt therebetween in such a manner that the molds' relative orientation is not changed during glass sheet engagement.
U.S. Pat. Nos. 4,662,925 and 4,666,492 to Thimons et al, 4,666,493 to Frank et al., and 4,666,496 to Fecik et al. teach a glass sheet shaping operation wherein heat softened glass sheets are deposited on the belt of an alignment device prior to shaping. The alignment device reorients the glass to the proper position between a pair of vertically aligned press faces which thereafter sandwich the glass sheet therebetween while supported on the alignment device belt.
U.S. Pat. No. 4,743,285 to McKelvey teaches a glass sheet bending apparatus in which heat softened glass sheets are shaped while being transported on a moving conveyor made up of a plurality of individual parallel belts. The contour of the conveyor varies from being generally flat, adjacent the apparatus inlet to being significantly curved adjacent the apparatus outlet. The softened glass sheet sags under its own weight to conform to the varying conveyor contour.